With regard to an assembling step of a semiconductor, wire bonding using a gold wire is the mainstream method. However, there is an increase in employment of bonding using a copper wire which is less expensive in material cost as compared to the gold wire.
However, in the bonding using the copper wire, copper and oxygen react with each other during formation of a ball by spark discharge, with the result that a copper ball of copper oxide is formed. Copper oxide has a higher hardness as compared to copper. Thus, at the time of bonding of the copper ball and a pad of an IC chip, the copper oxide may cause damage under the pad.
Further, discoloration and eccentricity may occur in the oxidized copper ball to cause adverse effects on bonding quality. A ball formed at a leading end of a wire by spark discharge is called a free air ball (hereinafter referred to as “FAB”).
In order to eliminate the adverse effects, it is necessary to prevent mixture of oxygen during a FAB forming step. Thus, in order to prevent oxidation of the ball, the FAB is formed mainly in an atmosphere of an inert gas, such as a nitrogen gas or an argon gas, or in an atmosphere of an oxidation-reduction gas, e.g., a nitrogen-hydrogen mixture gas. In the following, the inert gas and the oxidation-reduction gas are generically referred to as an oxidation prevention gas.
FIG. 11 is a perspective view for illustrating a positional relationship among a gas confining tube, a capillary, and a spark rod in a related-art wire bonding device. FIG. 12 is an explanatory view for illustrating a positional relationship among a capillary, a clamper, a gas discharging device including a spark rod, a jig tool, and the like of a wire bonding device in another related-art wire bonding device.
FIG. 11 is a perspective view for illustrating the capillary, the spark rod, and a periphery of those in the wire bonding device disclosed in Patent Literature 1. There is disclosed a wire bonding device configured to form a ball in an atmosphere of the oxidation prevention gas through use of the gas confining tube.
As illustrated in FIG. 11, the related-art wire bonding device disclosed in Patent Literature 1 includes a capillary 3 configured to draw out a copper wire 74 from a leading end 3a, a gas confining tube 85 having an upper opening 86 and a lower opening 87 formed so as to allow the leading end 3a of the capillary 3 to pass therethrough, and a spark rod 5 arranged in the gas confining tube 85 and configured to perform spark discharge with respect to a leading end portion 74a of the copper wire 74 drawn out from the leading end 3a of the capillary 3.
An atmosphere of the oxidation prevention gas is formed while allowing the oxidation prevention gas to flow through the gas confining tube 85, and spark discharge is performed through a leading end 5a of the spark rod 5 with respect to the leading end portion 74a of the copper wire 74, thereby forming the FAB.
As described above, in the related-art wire bonding device of FIG. 11, in order to prevent mixture of oxygen in the FAB forming step, while the oxidation prevention gas is allowed to flow through the gas confining tube 85, the leading end of the capillary 3 is inserted into the gas confining tube 85 through the upper opening 86, and spark discharge is performed through the leading end 5a of the spark rod 5 with respect to the leading end portion 74a of the copper wire 74 drawn out from the capillary 3, thereby forming the FAB in the atmosphere of the oxidation prevention gas.
Further, the applicant has proposed the invention of Patent Literature 2 according to FIG. 12 as another related-art bonding device.
FIG. 12 is an explanatory view for illustrating a positional relationship among the capillary, the clamper, and the gas discharging device including the spark rod (electric discharge electrode) in the wire bonding device described in Patent Literature 2. There is described a wire bonding device configured to form a ball in an atmosphere of the oxidation prevention gas through use of gas discharging tubes.
That is, as illustrated in FIG. 12, the related-art wire bonding device described in Patent Literature 2 includes a capillary 3 configured to draw out a copper wire 74 from a leading end, and a clamper 4 configured to clamp and release the wire 74 in conjunction with upward and downward movements of an ultrasonic horn 2 serving as a bonding arm.
Further, under the capillary 3, a gas discharging device 110 is arranged. In the gas discharging device 110, there are arranged a pair of gas discharging tubes including a first gas discharging tube 111 and a second gas discharging tube 120. The first gas discharging tube 111 has a cutout portion 111a formed at a leading end thereof so as to allow a leading end of the capillary 3 to be inserted therethrough, and is capable of discharging the oxidation prevention gas inside. The second gas discharging tube 120 has a cutout portion 120a formed at a leading end thereof, and is capable of discharging the oxidation prevention gas inside. The pair of gas discharging tubes is arranged so that the respective cutout portions 111a and 120a are opposed to each other. In the first gas discharging tube 111, a spark rod 5 is arranged. The respective cutout portions 111a and 120a arranged opposed to each other form an opening which allows insertion of the capillary 3 between the pair of gas discharging tubes.
Further, the first gas discharging tube 111a and the second gas discharging tube 120a have a gap between the respective leading ends, and the gas discharging device 110 includes position adjustment means 130 for adjusting a size of the gap.
Therefore, the wire 74 passes between clamping surfaces of an opening and closing mechanism of the clamper 4, passes through a hole formed in the capillary 3, and is drawn out from the leading end of the capillary 3. Further, under the capillary 3, the gas discharging device 110, which is capable of discharging the oxidation prevention gas inside, is arranged, and the gas discharging device 110 includes the spark rod 5. Thus, under a state in which both the leading end of the capillary 3 and the leading end of the wire 74 are inserted in the gas discharging device 110 through the respective cutout portions 111a and 120a of the first gas discharging tube 111 and the second gas discharging tube 120, that is, in the oxidation prevention gas atmosphere formed by the gas discharging device 110, a FAB 75 is formed at the leading end of the wire 74 by spark discharge.